The present invention generally relates to communications systems, and more specifically, to methods and systems for detecting optical link performance in a hybrid fiber coax path.
Fiber optic and coaxial cables are utilized to transmit signals along communications systems, such as a cable television systems. Cable television systems typically include a headend section for receiving satellite signals and converting the signals to a format appropriate for distribution over fiber optic and coaxial cables. The signals can then be converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters can be distributed throughout the cable system for splitting and transmitting optical signals, and optical receivers can be provided for receiving and converting the optical signals to electrical signals that can be further transmitted along branches of the system over coaxial cable rather than fiber optic cable.
The fiber optic and coaxial path in communications systems, referred to herein as a hybrid fiber/coax (HFC) path, has conventionally been employed in a forward manner to transmit high frequency (e.g., 50-750 MHz) cable television signals to subscribers. More recently, however, the HFC path has been used to carry information in a reverse manner, from a subscriber to a cable provider. The reverse HFC path typically operates at lower frequencies (e.g., 5-40 MHz) than those at which the forward path operates. The reverse path communication allows subscriber-generated signals, such as data, video, or audio signals, to be processed by the cable provider, enabling interactive services such as pay-per-view or video-on-demand programming.
Important in many communications systems, including cable television systems, is the need to provide reliable service with little interference. To this effect, an indication of signal quality is generally desirable. A cable service provider""s ability to effectively monitor the system""s signal quality, in both the forward and reverse path, can result in more efficient service having fewer disruptions and less down time because the provider may be able to more promptly locate and correct problems, ensure that minimum performance standards are being met, and gather historical data for predicting future performance of the system.
Most conventional coaxial forward and reverse paths utilize analog signals transmitted over coaxial cables. Typical monitoring of these paths offers no integral indicator of signal quality. The only indicator of signal quality at the receiver end of the path is whether the analog signal is received or not. Sophisticated external testing equipment and methods, such as spectrum analysis, are generally utilized to monitor and measure typical analog phenomenon, such as carrier to noise ratio and distortion.
Recently, devices for digitizing portions the reverse path have been made. For example, it is known to digitize the signal transmitted over the fiber optic link portion of the HFC path between the headend and a node. In the reverse path, a node receives analog signals from a subscriber, converts the analog signal to a digital signal, and transmits the digital signal over fiber optic cable to the headend, which converts the digital signal back to analog for transmission to the service provider.
A number of benefits result from digitizing the analog signal and transmitting the digital signal over a fiber optic cable, also referred to as an optical link. For example, unlike analog signals, digital signals can travel long distances (e.g., 60 km) as easily as shorter distances (e.g., 20 km) without any increase in signal deterioration, signal distortion, and signal-to-noise problems. Furthermore, digital signals can be recovered more easily at lower powers than their analog counterparts and can be implemented using readily available parts.
However, performance monitoring of an optical link in a digital HFC path requires expensive external test equipment which is not cost effective to utilize. Generally, therefore, an unsatisfied need exists in the industry for a low-cost method for monitoring digital performance of an optical link in a hybrid fiber coax path without the use of external monitoring equipment.